The phthalate portion of diethylhexyl phthalate (DEHP) does not bind to macromolecules, but a portion of the ethylhexyl chain does (in vivo). Phthalate was found in each of 200 samples of human urine; ubiquitous exposure was confirmed. Metabolism of DEHP involves extracellular, microsomal, cytoplasmic and mitochondrial enzymes; over 24 metabolites have been identified in the rat. Rats are very poor model for man. The enzymes that oxidize monoethylhexyl phthalate in rodents differ from those in primates both in positional and in stereo-specificity. Both the metabolites and the metabolic process itself may be involved in the biological activity of DEHP.